Assume O_DIRECTORY for opendir

Assume O_DIRECTORY for opendir

This patch assumes O_DIRECTORY works as defined by POSIX on opendir
implementation (aligning with other glibc code, for instance pwd).  This
allows remove both the fallback code to handle system with missing or
broken O_DIRECTORY along with the Linux specific opendir.c which just
advertise the working flag.

Checked on aarch64-linux-gnu, x86_64-linux-gnu, i686-linux-gnu,
sparcv9-linux-gnu, sparc64-linux-gnu, powerpc-linux-gnu, and
powerpc64le-linux-gnu.

	* sysdeps/posix/opendir.c (o_directory_works, tryopen_o_directory):
	Remove definitions.
	(opendir_oflags): Use O_DIRECTORY regardless.
	(__opendir, __opendirat): Remove need_isdir_precheck usage.
	* sysdeps/unix/sysv/linux/opendir.c: Remove file.